A spectrograph is a device for separating electromagnetic radiation of very short wavelengths (including visual light) into its spectral components. Optical spectrographs are used for chemical analysis, with applications in industry, medicine and science.
Spectrographs contain dispersive elements, such as prisms and diffraction gratings. Generally, modem spectrographs use diffraction gratings to analyze light. Spectrographs, to work efficiently, also contain optical elements to collect light for the prism or diffraction grating and to concentrate light onto a detector. A slit may be included to block light of unwanted wavelengths from the detector.
A common form of a spectrograph has a combination of prisms or a planar diffraction grating with optics for collecting and focusing light. The optics form the light into a mostly parallel beam to impinge onto a dispersive element. Separate optics are used for focusing light onto a slit or detector after dispersion by the prism or grating. Such optics may consist of mirrors, lenses or a combination of mirrors and lenses and may be used with entrance and exit slits as a monochromator or with a movable or array detector as a spectrograph. When a single mirror or a simple lens with spherical surfaces is used for collecting light, and a similar optical element is used for focusing light onto a slit or detector, a typical focal ratio is f/3.5. A focal ratio or f-number (represented by f/#, in which # shows a ratio) is the ratio of the distance between the source and the entrance pupil (aperture) to the diameter of the entrance pupil. A smaller focal ratio indicates better light-collecting ability.
Some spectrographs combine the functions of light collection, dispersion and concentration in a concave diffraction grating. The use of diffraction gratings in such spectral analyses began in the Eighteenth Century. The early gratings were planar, due to the difficulty of ruling concave gratings. Such gratings were used with a pair of telescopes, one for collimating light and one for viewing (early spectrographs required visual matching of spectra).
Concave diffraction gratings became popular after the invention of holographically-recorded diffraction gratings. Concave diffraction gratings are described in, for example, U.S. Pat. Nos. 3,628,849; 3,930,728; and 5,052,766.
Concave gratings, although capable of the combined function of collecting, dispersing and concentrating light, have certain limitations. The solid angle of light that can be collected by such gratings is relatively small. It is difficult to record gratings with a small focal ratio. For a concave grating, the focal ratio corresponds to the ratio of the distance from the source to the concave grating divided by the diameter of the light impinging of the concave grating. A smaller f/# indicates greater light collection capability. Most concave gratings handle a light cone with a focal ratio of about f/3. Even the "fastest" (i.e., with the smallest focal ratio to have the best light collection capability) gratings have a focal ratio of approximately f/2.
During manufacture of concave holographic gratings, the choice of recording beam parameters provides certain degrees of freedom for correcting aberrations in the image of the source at the detector. These degrees of freedom include recording wavelength, location of recording points, an aspheric surface on the grating blank and aberrations introduced into the recording beams. Using aspheric lenses is advantageous over using aspheric concave gratings. The concave grating surface and recorded hologram are coincident, which reduces the ability to use the aforementioned parameters for aberration correction. Since these degrees of freedom are limited, there are resulting residual aberrations. The magnitude of these aberrations increases with decreasing focal ratios, resulting in a limitation on the minimum focal ratio. The wide spread in angle of incidence for light onto a concave grating with a low focal ratio causes variation in diffraction efficiency, imposing an additional limitation on the minimum focal ratio.
Many spectroscopic measurements are made with weak signals. Therefore, to achieve good analysis, it is desirable to collect as many photons as possible. In certain spectral regions, and in all spectral regions for very weak signals, the measurement will be limited by noise generated in the detector. For these reasons, it is preferred to collect light from as large a solid angle as possible and to transfer this light onto as small a detector as possible. Since the product of the area of a source and the solid angle of the collected light remains constant within an optical system, to use a small detector, a large solid angle of light must not only be collected but also focused onto the detector. In addition, aberrations must be low to result in high image quality at the detector.
The light collection capability of mirrors is typically f/6.7, for example, as in the Perkin-Elmer ICP-OES spectrograph (see Barnard, Thomas, et. al., Anal. Chem. 65, 1225 (1993)) input optics. For typical fast mirror systems (e.g., Jobin-Yvon H-10, Jarrell-Ashe Model 82-410), the focal ratio is about f/3.5. Lenses for light collection and focusing are capable of smaller focal ratios than are mirrors. The Datta collection lens (see Datta, Sunil, Indian Journal of Pure and Applied Physics 22, 667 (1984)), a cemented two-element lens, has a focal ratio of f/3.65. The greatest collection capability in current commercial instrumentation, obtained with multi-element lenses in Kaiser Optical Systems Holo-Spec f/1.8i VPT SYSTEM, is f/1.8. Simple, single-element collecting and camera lenses are described by Eastman Kodak Company (see. U.S. Pat. No. 4,895,445 issued to Granger). The focal ratios in the Granger patent, determined from the scale of the drawings, appear to be relatively large.
Improvement in the optics for collecting light in a spectrometer can be found in the literature. U.S. Pat. No. 5,011,284 (Tedesco et. al.) discloses the use of an aspheric lens for collecting Raman scattered light for diffraction by transmission gratings on a prism. However, transmission gratings are relatively complex. Also, in the device of Tedesco et al. a relatively large light detector is still needed.
High image quality requires maintaining the specificity of light at different wavelengths and a high ratio of signal to noise by concentrating light onto a small detector because a large detector results in more noise. Prior art spectrographs use relatively complex optical system and yet have relatively large focal ratios. What is needed is a spectrograph with relatively simple optical elements, having a small focal ratio, and capable of focusing light of different wavelengths onto a relatively small detector.